It would presumably be the same it was essentially just a connection to the ISP so it is the same kinds of attack vectors browsing the wider web (arguably less since you presumably could call from random numbers and/or change ip address easily). The only one big thing I could think or is that since plain old telephone lines (POTS) were often directly interconnected (party lines/multiple phones to one line), someone could more easily pickup/tap the line to eavesdrop compared to broadband. USB modems do exist I believe 56k is the fastest you can find (iirc there was some FCC regulation limiting to 53k to prevent telco issues going faster, I believe 56-64k was the technical limit), but if you have a 2G phone that supports Circuit Switch Data you can use that and try out the magic of WML cards (specially formated html pages to run on phones) making up a WML deck / WAP site (mobile version of the site). There is a list of WAP sites here that may still work http://pubquizhelp.com/mobile/bestwap.html